In response to growing demand for expansion of data transmission volume, digital coherent techniques have been widely spreading to achieve high-speed high-capacity optical communications. With a digital coherent technique, a received light signal is detected using a local oscillation light beam, and digital processing is applied after optical-to-electrical conversion of the detected light signal to compensate for waveform distortion generated on the optical transmission path. Since individual chromatic dispersion compensators as well as optical amplifiers for compensating for the insertion loss, which have been required in a conventional technique, are omitted, the system can be downsized and stabilized, while achieving cost reduction.
For a next-generation optical transponder equipped with a digital signal processor, adaptive modulation schemes are discussed. In adaptive modulation, a bandwidth or a bit rate of a network is selective and the system will operate with a modulation scheme suitable to the selected bit rate. However, in reality, it is difficult to use appropriately adaptive modulation because the spectrum width expands upon increase of a baud rate responsive to an increased bit rate. Besides, baud rate is limited due to limit in speed of a digital-to-analog converter (DAC), and it cannot be increased beyond the limit of the DAC speed.
An optical communication technique capable of maintaining a transmission quality and suppressing an increase in power consumption is desired when performing adaptive modulation in accordance with a data transfer condition such as a channel spacing or a bit rate.
A new modulation scheme, 4-dimensional m-ary amplitude, n-ary phase shift keying (4D-mAnPSK) is proposed. See, for example, Japanese Laid-open Patent Publication No. 2017-513347. It is proposed to use, for example, 4D-2A8PSK and 4D-2A16QAM in place of conventional DP-8QAM and DP-16QAM.